Post-Newtonian gravity and Gaia-like astrometry. Effect of PPN $γ$ uncertainty on parallaxes

Abstract: Relativistic models of light propagation adopted for high-precision astrometry are based on the parametrised post-Newtonian formalism, which provides a framework for examining the effects of a hypothetical violation of general relativity on astrometric data. Astrometric observations are strongly affected by the post-Newtonian parameter $\gamma$ describing the strength of gravitational light deflection. We study both analytically and numerically how a deviation in the PPN parameter $\gamma$ from unity, which is the value predicted by general relativity, affects the parallax estimations in Gaia-like astrometry. Changes in the observable quantities produced by a small variation in PPN $\gamma$ were calculated analytically. We then considered how such variations of the observables are reflected in the parallax estimations, and we performed numerical simulations to check the theoretical predictions. A variation in the PPN $\gamma$ results in a global shift of parallaxes and we present a formula describing the parallax bias in terms of the satellite barycentric distance, the angle between the spin axis and the direction to the Sun, and the PPN $\gamma$ uncertainty. Numerical simulations of the astrometric solutions confirm the theoretical result. The up-to-date estimation of PPN $\gamma$ suggests that a corresponding contribution to the Gaia parallax zero point unlikely exceeds 0.2 $\mu$as. The numerical simulations indicate that the parallax shift is strongly dependent on ecliptic latitude. It is argued that this effect is due to an asymmetry in the Gaia scanning law and this conclusion is fully validated by additional simulations with a reversed direction of the precession of the spin axis around the direction to the Sun.